On modern vehicles (automobiles, trucks, motorcycles, etc.) a sensor making it possible to determine the speed of the vehicle is provided. This sensor is imposed by most legislations to allow the driver to ascertain and control the speed of his vehicle. This sensor, however, is also used for the correct operation of electronic devices that use this data item, such as for example a speed regulator (cruise control device).
If the speed sensor fails, the driver of the vehicle does not know its speed (hence a possible risk of excess speed) and the onboard devices using this data become inoperative. Such devices are increasingly numerous on modern vehicles. Consequently, the information given by the speed sensor is becoming increasingly important. Thus, for example, the speed sensor makes it possible generally to determine the distance traveled by the vehicle. It therefore becomes impossible, in the event of failure of the speed sensor, to plan the maintenance operations that are determined according to the mileage.
Nowadays, to alleviate a failure of the speed sensor, it is known practice to provide a second sensor. The latter then takes over from the first when the first is defective. This solution is satisfactory except that its cost is relatively high. It may therefore be reserved only for top-of-the-range vehicles.